Paste for the vibration filling of foam-structure and fiber-structure electrode plates for galvanic cells

ABSTRACT

A paste for vibration filling of foam-structure or fiber-structure electrode plates for galvanic cells, particularly for electrical accumulators, said paste having an active substance content of about 25 to 60 percent by volume, a maximum active particle size of about 0.04 mm and in an unvibrated state a yield value of approximately 10 to 120 Pa and a plastic viscosity of about 1.0 to 1.0 Pas at 20° C. The paste is preferably set in vibration with a frequency of about 40 to 125 Hz and an amplitude of about 0.1 to 1.5 mm by a vibration transmitter fitted inside a tank containing the paste.

This is a divisional of application Ser. No. 07/350,403, filed May 11,1989, now U.S. Pat. No. 4,974,644.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method for filling foam-structure orfiber-structure electrode plates with pastes of active substance forgalvanic cells, particularly for electrical accumulators, as well as toa composition of the pastes of active substance.

Electrode plates having a foam or fiber structure are being increasinglywidely used because they present advantages compared to the cheap castand expanded metal grids as regards electrical load capacity and usefullife. The fibers of these plates may consist entirely of metal, such asnickel-plated steel wool for example, or they may be formed bymetallization of plastic non-woven fabrics, according to U.S. Pat. No.3,560,262 for example.

Foam-structure plates, which are also frequently known as reticulatedplates, can be produced by the metallization of open-cell foam materialssuch as carbonized foam materials according to Great Britain patentspecification No. 1,211,428 for example, or of foam materials accordingto German Published, Unexamined Patent Application (DE-OS) 2,427,422 orby foaming a plastic material containing a metal powder and subsequentpyrolysis according to European Patent Specification (EP-PS) 87,160. Forreasons of production, the pores of these reticulated plates formpolyhedra of approximately spherical shape and vary far less in shapeand size than the pores of plates which consist of non-woven textilematerials. Due to the gas bubble sizes dictated by the processtechnology, the pore diameter of the reticulated plates is much greaterthan that of sintered plates produced by powder metallurgy, and, aconsiderable reduction in the thickness of the reticulated plates isgenerally undertaken by rolling or pressing. The plastic fraction isalmost always removed from the reticulated plates by pyrolysis, so thatall-metal plates with no plastic fraction are obtained in the finalresult.

The following classification of electrode plates categories by poresizes and pore shapes can be made:

a) Very large pores in the millimeter range in the form ofbi-dimensional holes regularly arranged: Cast grids, expanded metalgrids, nets.

b) Large pores, polyhedra approximating to spherical shape, porescross-linked three-dimensionally, narrow pore-size distribution:Reticulated (foam) plates.

c) Medium-sized pores, anisodimensional (widely different dimensions ofa pore in different physical directions), pore delimitation by partiallyelongate bars, three-dimensional cross-linkage of the pores: fiberplates and compressed foam materials.

d) Small pores, three-dimensionally cross-linked, pore delimitation bysintered necks, approximately 60% of the total pore volume in the formof pores in the range 4 to 15 micrometers (Falk and Salkind, AlkalineStorage Batteries, Publisher J. Wiley 1969, Page 122): powdermetallurgical sintered plates.

Various techniques which are oriented by the above enumerated porecharacteristics, have been developed for the introduction of the activesubstance by filling or impregnation.

The filling of the plates mentioned under category a), which areprovided with two-dimensional, optically transparent hole patterns, isgenerally done by pressing in a paste of active substance mechanically,as for the electrodes of lead accumulators, where this technology hasbeen applied to cast grids and expanded metal grids. Rollers or doctorblades are used as pressure-imparting elements. Since the activesubstance must be sufficiently firm not to flow out of the holes afterpasting, but sufficiently plastic to be still spreadable, high viscositythixotropic pastes are used (e.g. German patent specification Nos.(DE-PS) 2,517,368 or (DE-PS) 2,602,904) which become sufficiently fluidunder the pressure of the roller or of the doctor blade.

For narrow-pored plates with a three-dimensional pore network, thismethod becomes more difficult the narrower the pores are. Only chemicalor electrochemical precipitation methods of embedding the activesubstances have therefore been successful for sintered plates, becausethe solutions can penetrate even into very small pores. However, themethods which operate with solutions suffer from disadvantages oftediousness, accompanying frame corrosion and also contamination of theprecipitation products by anions of the initial compound, nitrate,sulphate or chloride ions being typical for example. The tediousness ofthe filling is due, in principle, to the much lower ion concentration inthe solutions as compared to the solids, so that much time is requiredto build up the desired high solid concentration of the active substancein the pore volume of the electrode.

Both the precipitation impregnations known for category d) and alsomechanical filling methods with suspensions and pastes have beendescribed for the foam-structure and fiber-structure plates mentionedunder categories b) and c). Naturally, the less expensive mechanicalmethods encounter increasing difficulty of realization with a reductionin the pore size, because suspensions and pastes do not penetrate into anetwork of fine pores as easily as solutions.

A filling with a paste of active substance with agitation is describedfor reticulated plates with large pores between 230 and 2,540micrometers in German patent specification No. (DE-PS) 1,596,023 (whichcorrespond to U.S. Pat. No. 3,287,164). The liquid is 30% caustic sodasolution. No information is given as to the viscosity of the paste, theagitation conditions (frequency, intensity, arrangement). However, thefilling of such large pores presents no problems.

The filling of a foam plate with a pore diameter of 100 to 500micrometers without metallization is described with similar terseness inGerman Published, Examined Patent Application (DE-AS) 1,108,759.According to this document an active substance placed in suspension ispressed in, optionally with agitation. However, according to thedescription the active substance suspension cannot be a paste.

An apparatus for the impregnation of reticulated plates (foam metal)with paste, likewise with no disclosure concerning the details of thepore size, is described in U.S. Pat. No. 4,217,939. The plate is guidedhorizontally on a perforated plate over a paste tank, in which the pasteis moved by stirrers and pressed upwards into and round the plate, thepaste being swept into the plate from above by reciprocating doctorblades. However, it is not immediately possible then to displace the airout of the pores of the plates by the paste. On the contrary, apreimpregnation of the plate with water is necessary for this purpose.The dilution of the paste with this water creates difficulties inadjusting the concentration of the paste.

The filling of fiber-plate electrodes with paste is described in aplurality of applications.

U.S. Pat. No. 3,262,815, corresponding to Great Britain patentspecifications No. 1,109,524, describes three different methods forintroducing a suspension of an active substance into a fiber plate: 1)working in mechanically, 2) depth filtration and 3) introducing theplate into a mechanically moved (agitated) bath, where the plate or thebath may be set in motion. None of these three methods operatessatisfactorily by itself, because the disclosure is directed at aprocess which should combine all three methods. Thus, after treatment inaccordance with method step 3) noted above, the plate should stillundergo an after-treatment according to method steps 2) and 1). Noinformation is given as to the mechanical characteristics of the paste;however, the treatment according to method step 3) alone clearly doesnot provide a satisfactory filling.

Then again, in a later application, German Published, Unexamined PatentApplication (DE-OS) 2,436,704, the same applicant refers to thebasic/process of working in a thixotrophic paste by pressure rollerapplication, as already described above for lead grids, except that themethod is applied to nickel fiber plates of 92% porosity and 1.7 mmthickness (Example 1). After filling, these plates are compressed to 0.9mm thickness. This high compression, which would reduce the porosity ofan unfilled plate from 92% to 85%, indicates that a paste filling intothe finished frame of final thickness clearly does not succeed, a doublepaste application being required even for the still uncompressed frameof 92% porosity.

German Published, Unexamined Patent Applications (DE-OS) 2,427,421(which corresponds to U.S. Pat. No. 3,877,987) and (DE-OS) 2,427,422(which corresponds to U.S. Pat. No. 3,926,671) refer in identical textsto the possibility of filling fiber plates with suspensions of activesubstance which are pourable and therefore highly fluid. The suspensionis poured onto the horizontally positioned plate and a vibratorelectrode arranged parallel to the frame, which is in communication withan ultrasonic generator, and in combination with vacuum ensures asubstantial filling, which however still requires completion by afollowing precipitation impregnation. According to the descriptions inthese applications, the suspensions of active substance are not pastes;nor is the term paste used.

The specifications discussed above refer to plates which must beclassified under a specific category a) to c) of the above-mentionedframe classification according to the pore size. However, a vibrationfilling with pastes under the influence of ultrasonic vibrations, andoptionally of vacuum, is disclosed by German Published, Examined PatentApplication (DE-AS) 1,287,663, which is claimed to be useful both forfiber plates of category (c) and also for sintered plates of category(d) produced by powder metallurgy. By this filling method the paste isapplied in a thin uniform layer onto the ultrasonic vibrator, and apressure element presses the paste layer together with the platesstrongly against the vibrator, a perforated foil being placed betweenelectrode plate and pressure element. No detailed information is givenas to the flow characteristics of the paste, except for the example,according to which the paste should have a consistency somewhat likespreadable butter.

The horizontal positioning of the plate, which is adopted for allfilling processes under the influence of vibrations, is consideredunavoidable because in the vibration field the dispersed solid has atendency to settle downwards, whereas the liquid moves upwards (SeeGerman Published, Examined Patent Application (DE-AS) 1,287,663).However, considerable disadvantages are associated with the horizontalpositioning. It is known for instance for the filled plate to stick veryfirmly to the pressure plate or to the surface of the vibrator, whichhas the result that special removal technologies were proposed (SeeGerman Patent Specifications (DE-PS) 1,210,417 for example) or that thecorresponding parts threatened by sticking are provided with anantiadhesion coating or with an antiadhesion foil. Furthermore, theproduction process according to the two last mentioned patents, wherebythe plate to be filled is sandwiched horizontally between two layers ofthe active substance and is then filled under pressure and the influenceof vibrations, is naturally also extraordinarily complicated.

To sum up, it can be stated that in the use of pourable suspensions,such as they are described in German Published, Examined PatentApplication (DE-AS) 1,108,759, German Published, Unexamined PatentApplication (DE-OS) 2,427,421, U.S. Pat. No. 3,262,815, U.S. Pat. No.3,877,987 or U.S. Pat. No. 3, 926,671 for example, a single fillingoperation does not effect an adequate filling of the pores, so that aplurality of impregnation passes or following impregnations arerecommended.

In the use of pastes of high solids content, i.e. pastes beingunderstood to mean, according to Rompp, Chemielexikon 8th Edition 1985,Volume 4, Page 3006, solid/liquid dispersions of doughy consistence, andtherefore not pourable suspensions, the difficulties known from U.S.Pat. No. 4,217,939 and German Published, Examined Patent Application(DE-AS) 1,287,663 as described above result.

An object of the present invention is to provide a method for thefilling of foam-structure or fiber-structure electrode plates with apaste of active substance which is simple, fast and can be performedwithout a major outlay of apparatus or effort.

A further object of the present invention is to provide a paste ofactive substance suited for the method of filling the electrode platesof the present invention.

Thus, in accordance with preferred embodiments of the present invention,the electrode plates to be filled are immersed in a tank filled with apaste of active substance, in which the active substance is set invibration. The paste in the tank has an active substance content ofabout 25 to 60 vol %, with a maximum granulometry of the activeparticles of about 0.04 mm and a yield value of about 10 to 120 Pa atabout 20° C. and a plastic viscosity of about 0.1 to 1 Pas at about 20°C. The viscosity particulars here refer to a paste which is not invibration. This is because a reliable viscosimetry is not known for avibrating paste.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side, sectional view schematically illustrating alternativeembodiments of an apparatus for performing an impregnation offoam-structure and fiber-structure electrode frames in accordance withthe method of the present invention;

FIG. 2A is a schematic view illustrating one embodiment of a vibrationplate which is provided with a U-shaped profile; and

FIG. 2B is a side, sectional view of an alternative embodiment of thevibration plate of FIG. 2A which is provided with a base part forsetting an immersion depth of a frame to be filled.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Concentrated suspensions belong to the class of plastic substances, thatis to say they have a yield value. The yield value assured hereinafteris defined as the smallest force per unit area (shear stress, tangentialstress) which is required to cause a flow. For an explanation of therheological terms and measuring methods, attention is drawn to thedocument entitled "Measurement of rheological characteristics", BulletinT 990 D-7309, Contraves AG Zurich, Pages 16 to 20 and to the publicationby A. Finke and W. Heinz entitled "On the determination of the yieldvalue of coarsely disperse systems", Rheological Aca, 1, (1961), 530 to538. The rheometrical verification of the paste characteristics can beperformed according to the above-mentioned literature with aviscosimeter.

The assignee company of the present application has found the rotaryviscosimeter Rotovisco RV 12 (Haake Company) with the measuring devicesNV and MVI successful. Shear rates of at least 100/s should be obtained.The return curve is recommended for evaluation, and goodthermostatization must be ensured. A so-called grindometer, in which apaste spread of decreasing thickness is assessed, is sufficient for themeasurement of the maximum granulometry.

The viscosity of the paste may be influenced not only by the quantityand the granulometry of the active substance, but may also be modifiedby the addition of other substances. The expert is familiar, forexample, with the addition of wetting agents (as disclosed by GermanPublished, Examined Patent Application (DE-AS) 1,108,759 for example),dispersing agents (as disclosed by German Published, Unexamined PatentApplication (DE-OS) 2,346,704 for example), expanders (as disclosed byGerman Published Examined Patent Application (DE-AS) 1,287,663 or),German Pat. Nos. (DE-PS) 2,517,368, (DE-PS) 2,602,904 for example),thickening agents (as described by German Published, Unexamined PatentApplication (DE-OS) 2,436,704,and U.S. Pat. No. 4,217,939 for example),but also carbon black (as disclosed by U.S. Pat. No. 3,262,815 forexample).

The granulometry of the particles of the paste of active substanceshould not exceed about 0.04 mm, because difficulties with the fillingmay otherwise arise in the case of fine pores. The paste may be producedfrom any kind of active substances known, provided they do not reactwith the dispersion liquid.

The electrode plates to be filled are immersed horizontally orvertically, generally hanging up by their terminal tab, into the tankfilled with the paste of active substance set in vibration, and aresimply withdrawn from it again in the same way after filling. The merepossibility of being able to fill the plaque completely by simplesuspension in the paste of active substance represents a quitesignificant simplification of the method compared to the methodshitherto known.

The paste of active substance is set in vibration in order to allow thepaste of active substance to penetrate into the pores of the frames.This is not restricted to the ultrasonic range. Mechanical agitators,which deliver high power at low cost, may also be used advantageously.The frequency range from about 40 to approximately 125 Hz has been foundparticularly successful, because these latter vibrations can be derivedparticularly favorably from the main frequencies generally in use. Theagitator should preferably be operated in the acceleration range fromabout 1 to 15 g. Acceleration b_(g) (in g) and frequency f and amplitudeA are related by the equation b_(g) =4π² f² A.

For the method according to preferred embodiments of the presentinvention, an apparatus 1 for performing the impregnations is ofextremely simple construction as shown in FIG. 1. The paste 2 of activesubstance is filled into a tank 3 and a plate 4 to be filled is immersedinto the liquefied paste 2 set in vibration. The vessel 3 or the plate 4to be filled is set in vibration by means of vibrator 5 or 5';respectively, that is to say agitated, in order to introduce thevibrations into the paste 2. However, in the case of large quantities ofpaste, such as must be used in production, problems arise with theagitation of correspondingly large tanks. In this case, the vibrationmay be transmitted to the paste by transmitters 6 which are locatedinside the tank 3. Such transmitters 6 generally consist of a plate 7which is driven by an exciter 8 arranged outside the paste bath. It isparticularly favorable to use a vibration transmitter 6 which consistsof a plate 9 having a U-shaped profile open at its end faces andreceiving the electrode plate 4 totally and without contact at least atthe sides as best shown in FIG. 2A. The sides of this U-shaped plate 9as shown in FIG. 2B are approximately as high as the frame 4 to befilled, which is introduced from above and a bottom edge of which may bebraced against a base part 10 of the U-shaped as illustrated in FIG. 2A,so that the immersion depth of the plate 4 is defined. This may beadvantageous if it is desired to prevent an electrode tab 11 from beingcontaminated with paste.

The amplitude with which the tank, the plate to be filled or thevibration transmitter vibrate, should be approximately between 0.1 to1.5 mm. Larger amplitudes do not generally produce an improved effect,and smaller amplitudes sometimes dictate longer impregnation times. Thedevelopment of standing waves on the surface of the paste bath may serveas an optical aid for the correct adjustment of frequency and amplitude.The direction in which the vibrations are introduced into the paste bathis not critical for the method. However, it is preferred to introducethe vibrations so that the wave front strikes the plate to be filled atright angles. The utilization of the wave energy is optimum in thiscase. This is achieved most simply by arranging the vibrationtransmitter parallel to the plate to be filled, as is already known perse from the cited prior art.

In the course of the method according to preferred embodiments, theempty plates are immersed individually or simultaneously in groups intothe paste bath with continuous vibration. As air escapes, the platesbecome filled after about 10 to 30 seconds. The dwell time may befurther prolonged up to 2 to 3 minutes with a gain in paste filling,i.e., in landing of the plate. After the plates are extracted from thebath the surplus paste is wiped off by pairs of rubber lips which arearranged above the paste bath. Cleaning by brushes is likewise possible.

The method makes it possible to fill large quantities of electrodeplates rapidly and without major outlay. The surprising fact here isthat, contrary to the opinion expressed in the literature, nosegregation of the paste bath results from the application of thevibrations.

EXAMPLE 1

About 480 g ZnO were mixed with about 128 ml water which contained about0.2% of a dispersing agent based on a sodium salt of a polycarboxylicacid of moderate degree of polymerization (Tamol PA, Manufacturer BASF)and about 2% sodium triphosphate, in a ball mill for four hours. Theflow curve of the resulting paste was measured with a rotaryviscosimeter (Haake Company Rotovisco with NV measuring device). Itshowed the typical picture of an ideally plastic so-called Binghamsubstance. The flow curve of the paste was fully described by the yieldvalue of about 24 Pa and by the plastic viscosity of about 230 mPas. Thepaste contained about 40 vol % ZnO.

A copper foam plate about 2 mm thick having a porosity of about 92% atabout 100μ mean pore size was used as electrode plates. The paste wasloaded into a prismatic vessel which was fastened on anelectromagnetically driven agitator (shaker). The paste was set invibration at about 50 Hz and about 0.5 mm amplitude and liquefied. Theliquid state was detectable by the development of standing waves on thesurface of the suspension. The copper foam frame was immersed verticallyinto the suspension. The emergence of air bubbles was then observed.About one minute after immersion the plate was removed, cleaned ofadhering surplus material by brushing and weighed. Evaluation showedthat about 98% of the pore volume was filled with paste.

EXAMPLE 2

About 405 g nickel hydroxide powder and about 12.5 g cobalt powder wereground with about 182 g of an approximately 5% aqueous sodiumpolyphosphate solution in a porcelain ball mill of about 1 litercapacity using about 540 g aluminum oxide grinding balls of about 16 mmdiameter at about 70 revolutions/min for about 16 hours. The pasteobtained in this way had a yield value of about 105 Pa and a plasticviscosity of about 300 mPas. The maximum granulometry measured by thegrindometer was about 23μ. The paste had an active substance content ofabout 36.7 vol % nickel hydroxide and about 0.5 vol % cobalt. Afterseparation from the grinding balls, the paste was transferred into asteel vessel which was fastened on an agitating table. The vibrationenergy introduced into the paste was adjusted so that the paste did notquite spatter on the surface, which was the case at a frequency of about40 Hz with an amplitude of about 1.25 mm. The electrode plate to befilled consisted of about 4 mm thick nickel-plated fiber plate ofpolypropylene non-woven with a porosity of about 85%. The average poresize, calculated from the fiber diameter and the weight of the samplebefore and after nickel-plating, was 78μ. The plate was supported by thewelded tab and remained submerged in the vibrated fluidized paste forabout one minute. On extraction, the plate was passed between rubberlips, which wiped off the still unsolidified surplus paste. By virtue ofthe set viscosity, the paste did not flow out of the filled pores of theplate. A filling of about 96% of the pore volume and an active substancecontent of about 1.4 g/ml void volume were arrived at from the weight ofthe empty and filled plate and from the dry weight after drying at about80° C. No variation of the concentration, or variation of the content ofwater or solid in the paste over the experimental period was found froma plurality of a consecutively performed impregnation experiments andfrom the determination of the water content of the paste.

EXAMPLE 3

About 600 g iron(III) oxide (Bayferrox 1370, Manufacturer Bayer AG) andabout 121.8 g of an approximately 2% aqueous sodium polyphosphatesolution (Kalgon 322, Manufacturer Benckiser-Knapsack), whichadditionally contained about 1.5 weight % polyvinyl alcohol, were mixedfor about 30 min in a laboratory mixer. The active substance content inthe paste thus produced was about 48.8 vol %. The flow curve correspondto a non-ideal plastic behavior, that is to say after the yield valuewas exceeded the shear rate in the range of small values did notincrease linearly with the tensile stress at first. From the linear partof the flow curve, a yield value of about 29 Pa and a plastic viscosityof about 760 mPas were calculated up to the shear rate of about 130/s.After the paste had been transferred into a tank, a perforated sheetmetal cage of U-shaped configuration open at end faces was inserted intothe tank, which was connected by a rigid sheet metal arm to anelectromagnetic vibrator fitted above the vessel. The perforated sheetmetal cage was made to vibrate at about 50 Hz and about 1 mm amplitudeand fluidized the paste of active substance. A plate about 1.5 mm thickmade of nickel-plated polypropylene non-woven according to Example 2 wasplaced in suspension in the cage by its tag connected to the top edge ofthe plate. After about a 30 second dwell time in the fluidized paste andwiping off the surplus, differential weighing revealed a virtuallycomplete filling of the plate.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only, and is not to be taken by way oflimitation. The spirit and scope of the present invention are to belimited only by the terms of the appended claims.

What is claimed:
 1. A paste having an active substance for vibrationfilling of foam-structure or fiber-structure electrode plates forgalvanic cells, said paste havingan active substance content in therange of from approximately to 25 to 60 percent by volume; maximumactive particle size of approximately 0.04 mm; and in the unvibratedstate, having a yield value in the range of about 10 to 120 Pa and aplastic viscosity in the range of about 0.1 to 1 Pa.s at about 20° C.;and said paste having a dispersing agent selected from the group ofwater-soluble salts of polyphosphoric acid.
 2. A paste of activesubstance according to claim 1, wherein the dispersing agent is awater-soluble alkali salt of polyphosphoric acid.
 3. A paste of activesubstance according to claim 1, which contains at least one of a cobaltpowder and cobalt compounds from oxide, hydroxide, bromate and phosphateclasses.
 4. A paste of active substance according to claim 1 whereinsaid active substance comprises nickel and cobalt, and wherein totalcobalt content amounts to up to about 12 atomic percent, relative tonickel, in the paste.